neutron instrumentation - neutron... · summary • neutron instrument concepts – time-of-flight...

Neutron Instrumentation

Ken Andersen

Oxford School on Neutron Scattering5th September 2019

Summary

• Neutron instrument concepts– time-of-flight– Bragg’s law

• Neutron Instrumentation– guides– monochromators– shielding– detectors– choppers– sample environment– collimation

• Neutron diffractometers• Neutron spectrometers

2

The time-of-flight (TOF) method

distance

time

Δt

Diffraction: Bragg’s Law


θλ sin2d=


2θ

θλ sin2d=

n=1

n’<1

incident

refracted

reflected

θ

Reflection: Snell’s Law

θ’

n=1

n’<1

incident

refracted

reflected

θ


θ’ θ’=0: critical angle of total reflection θc

n=1

n’<1

incident

refracted

reflected

θ



Nb/πλθ

2θ1cosθ2π

bNλ1n'

n'nn'cosθ

c

2cc

2c

=⇒

−≈

−=

==

n=1

n’<1

incident

refracted

reflected

θ



Nb/πλθ

2θ1cosθ2π

bNλ1n'

n'nn'cosθ

c

2cc

2c

=⇒

−≈

−=

==for natural Ni,

θc = λ[Å]×0.1°

Qc = 0.0218 Å-1

Courtesy of J. Stahn, PSI

Neutron Supermirrors


18

d1}

λ 1

λ 2

λ 3

λ 4

λ c

d2} d3

} d4

}

Reflection: θc(Ni) = λ[Å] × 0.10°

Multilayer: θc(SM) = m × λ[Å] × 0.10°


19

}

λ 1

λ 2

λ 3

λ 4

λ c

} } }

Reflection: θc(Ni) = λ[Å] × 0.10°

Multilayer: θc(SM) = m × λ[Å] × 0.10°

“m-number” Supermirror critical angle d1

d2

d3

d4

State-of-the-art Supermirrors

20

Mirrotron

State-of-the-art Supermirrors

21

Mirrotron

Swiss Neutronics

WISH @ ISIS

Neutron guides

Swiss Neutronics guides for NIST

Distribution by GuidesNeutron Guides: Creating space for instruments

Background Reduction

• Distance:– move away from fast-

neutron source ~ 1/R2

Guides can be used to reduce background

Background Reduction

• Distance:– move away from fast-

neutron source ~ 1/R2

• Curved Guides: – avoid direct line-of-sight– avoid gammas– avoid fast neutrons

Guides can be used to reduce background

Focusing

26

Converging guide increases flux, but increases divergence

Guides can also be used to increase flux

Shielding

• Shielding functions: – allow safe operation– keep background down– reduce activation

• Radiation components:– Slow neutrons– Fast neutrons– Gammas

27

Shielding: radiation units and numbers

Unit Description

Curies Decay rate: 3.7×1010 Bq (decays/s)

Gray Energy dose: J/kg

Sievert Biological effect of radiation

Roentgen, rad, rem – legacy units

28

Average Person Dose

Radon 2 mSv/year

Cosmic 0.28 mSv/year

Terrestrial 0.28 mSv/year

Internal 0.4 mSv/year

Medical x-rays 0.39 mSv/year

Nuclear medical 0.14 mSv/year

Consumer products 0.1 mSv/year

Other 0.03 mSv/year

Total 3.62 mSv/yearSource: Günter Muhrer, Los Alamos, New Mexico

Examples Dose

Dental x-ray 10 μSv

Abdominal CAT scan 10 mSv

Airline flight crew members 10 mSv/year

50% probability of death 5 Sv

Neutron experimental halls < 3 μSv/hour× 2000 hours: < 6 mSv/year

ISIS-TS2

Shielding: slow & fast neutrons

29

slow epithermal fast

• Slow neutrons– easy to absorb (few mm B, Cd, Gd, ..)– emit gammas when absorbed– easily detected

• Fast neutrons– can require several m of material for

absorption– alternatively, thermalise using

hydrogenous material (e.g. wax or polyethylene) then absorb

– both require multiple (10s) of collisions

– visualise as a gas, filling both solids and air

– difficult to detect• Gammas

– high-energy photons– absorbed by any material– absorption length scales inversely

with density: Pb 11.3 kg/m3, steel 7.9 kg/m3, concrete 2.4 kg/m3

cold thermal

ESS Target Monolith: 11m diameter

WISH @ ISIS-TS2

10-1410-1110-910-710-210-1

Tim

e [s

]

10-12

10-9

10-15

10-6

10-3

1

103

10-6

Particle Physics

Spin-echo

Imaging

Reflectometry

10-10 10-12 10-1510-810-5 10-6

Backscattering

TOF & Crystal Spectroscopy

SANSDiffraction

Ener

gy [m

eV]

Length [m]

10210-1 110-3 10110-2

103Wavevector Q [Å-1]

10-3

10-1410-1110-910-710-210-1

Tim

e [s

]

10-12

10-9

10-15

10-6

10-3

1

103

10-6

Particle Physics

Spin-echo

Imaging

Reflectometry

10-10 10-12 10-1510-810-5

Visib

le

Ligh

t

InfraredLight

FTIRRaman

Brillouin

Dynamic Light Scattering

Laser PCS

10-6

Inelastic X-ray Scattering

NM

RµS

R

Diel

ectr

ic S

pec.Backscattering

UV & X-ray FEL: Pump & Probe studies

NMR Imaging Triangulation NMR

X-ray Imaging SAXS, X-ray Refl. X-ray Diffraction EXAFS & XANES

MicroscopyOptical Microscopy

Scanning TechniquesSNOM AFM STM

TOF & Crystal Spectroscopy

Infr

a-re

d

SANSDiffraction

Ener

gy [m

eV]

Length [m]

10210-1 110-3 10110-2

103

X-ray Spec.

X-ray PCS

Wavevector Q [Å-1]

10-3

X-rays

NMR

Optical Nanoscopy Electron Microscopy

Diffractometers

• Measure structures (d-spacings)• Very general method:

– crystals– powders– polycrystalline materials– liquids– large molecules or structures– surfaces

• Ideal diffractometer: – measure ki of each incident neutron– measure kf of the same neutrons

33

𝑘𝑘𝑖𝑖𝑘𝑘𝑓𝑓

Diffractometers

• Measure structures (d-spacings)• Very general method:

– crystals– powders– polycrystalline materials– liquids– large molecules or structures– surfaces

• Real diffractometer: – measure ki or kf of the beam

• time-of-flight• Bragg diffraction

– assume ki = kf

34


Powder diffractometers

Polycrystal

θλ sin2d=d

Q π2=

• Measure crystal structure using Bragg’s law– Rietveld refinement

• Large single crystals are rarely available

distance

time

sample

detector

Δt

θλ sin2d=

Time-of-flight (TOF) Method

time

Δt

Time-of-flight Resolution

( )22

2

∆

∂∂

+

∆

∂∂

=∆ θθ

λλ

dddθλ

sin2=d

time

Δt

Time-of-flight Resolution

( )22

2

∆

∂∂

+

∆

∂∂

=∆ θθ

λλ

dddθλ

sin2=d

To improve the resolution:• increase the length: long guide• move to a sharper moderator• reduce the beam divergence

WISH @ ISIS TS2

Time-of-flight (TOF) Method

Copper 200Graphite 002

d-spacingGermanium 333 1.089 ÅCopper 200 1.807 ÅSilicon 111 3.135 ÅGraphite 002 3.355 Å

Crystal Monochromators

Pulsed source time structures (λ=5Å)

43

log(

Inte

nsity

@λ=

5Å)

0 20 40 80 100 120 time (ms)

1

0.1

0.01

ISIS-TS1 128kW ISIS-TS2 32kW100μs

0

short pulse

ILL 57MW

Pulsed source time structures (λ=5Å)

44

log(

Inte

nsity

@λ=

5Å)

0 20 40 80 100 120 time (ms)

1

0.1

0.01

ISIS-TS1 128kW ISIS-TS2 32kW100μs

0

short pulse

ILL 57MW

D1B @ ILL

Constant-Wavelength Diffraction

Rietveld refinement

2θDebye-Scherrer cone

The Diffractometer Family

• Powder diffraction– chemical crystallography– disordered materials– engineering: strain scanning

• Single-crystal diffraction– magnetic ordering– diffuse scattering– large unit cells – protein crystallography

• Small Angle Neutron Scattering (SANS)– soft matter – macromolecules in solution– nanomaterials

• Reflectometry– surfaces and interfaces– both planar and in-plane structures

46

Sample environment

• neutron penetration: good• sample volume: bad• range as varied as the science

– magnetic fields– low temperatures– high temperatures– pressure cells (TiZr)– sample changers– stress rigs– in-situ chemistry– flow cells– Langmuir troughs– …

47

ILL adsorption trough

ISIS Cryomagnets

SNS pressure cell

ILL 15mK dilution cryostat

Neutron Spectroscopy

• Excitations: vibrations and other motions– lattice vibrations– magnetic excitations– quasi-elastic scattering: diffusion & relaxation

48



• Ideal spectrometer: – measure ki of each incident neutron– measure kf of the same neutrons

49




• Real spectrometer: – measure ki and kf of the beam

• time-of-flight• Bragg diffraction• Larmor precession

– Fix ki and scan through kf – “direct geometry”– Fix kf and scan through ki – “indirect geometry”

50


distance

time

chopper

detector

sample

0

Direct geometry: fix ki by chopper phasingscan through kf by time-of-flight

Chopper Spectrometers

Disk choppersFermi choppers

f < 300 Hz

Δt > 10μs

f < 600 Hz

Δt > 1μs

Neutron Choppers

• General-Purpose Spectrometers– Incident energy ranges from 1meV to 1eV

• Huge position-sensitive detector arrays– Single-crystal samples

Chopper Spectrometers

3He gas tubesn + 3He 3H + 1H + 0.764 MeV>1mm resolutionHigh efficiencyLow gamma-sensitivity3He supply problem

Scintillatorsn + 6Li 4He + 3H + 4.79 MeV<1mm resolutionMedium efficiencySome gamma-sensitivityMagnetic-field sensitivity

Detectors

10B detectorsn + 10B 7Li + 4He + 0.48 MeVmassive development programmenone yet in operationmany different types

Detectors

inclined blades

perpendicular bladesboron layer thickness limited to ~ 1 μm

=> ~ 5% efficiency

Direct-Geometry Kinematics

56

ki

kfQ

2θ

𝑄𝑄 = 𝑘𝑘𝑖𝑖 − 𝑘𝑘𝑓𝑓


57

ki

kfQ

2θ

𝐸𝐸 =𝑝𝑝2

2𝑚𝑚=ℏ2𝑘𝑘2

2𝑚𝑚



58

ki

kfQ

2θ

𝐸𝐸 =𝑝𝑝2


2𝑚𝑚



59Momentum transfer Q

ki

kfQ

2θ

Ener

gy T

rans

fer =

Ei-

E f

𝐸𝐸 =𝑝𝑝2


2𝑚𝑚


distance

time

detector

sample

0

Indirect geometry: fix kf – usually by analyser crystalsscan through ki by time-of-flight

analyser

Alternative to Direct Geometry

Indirect-Geometry Kinematics

61Momentum transfer Q

ki

kfQ

2θ Ener

gy T

rans

fer =

Ei-

E f

Vibrational Spectroscopy

62

TOSCA@ISIS

Density-of-states measurements

θθλλ

θλ

∆+∆

=∆

⇒

=

cot

sin2

dd

d

0sincoscot

2

→=

→

θθθ

πθ

Use single crystals in as close to backscattering as possible to define kf. Scan through ki with as good energy resolution.

High Resolution 1: Backscattering

Backscattering

BASIS@SNS Si111 3μeV

OSIRIS@ISIS PG002 25μeV

ϕ

θ

B0 B1

L0 L1

Pz

π-flipper

High energy resolution < 1 μeV

Larmor precessions encode energy transfer

High Resolution 2: Neutron Spin Echo

• Single-crystal excitations• Very flexible• Measures a single point

in -E space at a time• Scans:

– Constant : Scan E at constant ki or kf

– Constant E: Scan in any direction

Q

Q

Q

Triple-Axis Spectrometers

Collimation

• TAS E and Q resolution adjusted by collimation

Soller collimator

Collimation

• TAS E and Q resolution adjusted by collimation• SANS resolution also adjusted by collimation

68

Soller collimator

Pin-holes separated by distance

> 0.1°5 cm 5 cm< 30 m

TAS with multiplexing

Top view

sample

31 channels75º angular range

kf = 3 Å-1 kf = 1.5 Å-1

Side view

IN20 flat-cone multi-analyser

Neutron Imaging

70neutron x-ray

Summary

• Neutron instrument concepts– time-of-flight– Bragg’s law

• Neutron Instrumentation– guides– monochromators– shielding– detectors– choppers– sample environment– collimation

• Neutron diffractometers– powder diffraction

• Neutron spectrometers– direct and indirect geometry time-of-flight– backscattering– triple-axis– spin-echo

• Imaging

71

Thank you!

725th June 2015

neutron instrumentation - neutron... · summary • neutron instrument concepts – time-of-flight...

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